1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image for use in developing an electrostatic latent image formed by an electrophotographic, electrostatic, or other recording process, a production method thereof, a resin particle dispersion for use in production of the toner, and an electrostatic image developer containing the toner.
2. Description on the Related Art
Methods such as electrophotographic processes and others that visualize image information through electrostatically charged images are currently used in a variety of fields. In an electrophotographic process, an image is visualized by forming an electrostatically charged image on a photoreceptor in electrostatic charging and photoexposing steps, developing the electrostatic latent image formed thereon with a developer containing a toner, and processing additionally through image-transferring and fusing steps to visualize the image. There are two kinds of developers for use in such systems: two-component developers consisting of a toner and a carrier and one-component developers employing only one magnetic or nonmagnetic toner. A kneading-pulverizing process, wherein a thermoplastic resin is melted and kneaded with a pigment, a charge controlling agent, and a releasing agent such as a wax or the like, and the resulting mixture is pulverized and classified after cooling, is commonly used as the method for producing toners. Inorganic or organic particles may be added to the toner if needed as an additive to the toner particle surface for improvement in fluidity and cleanability.
Recently, copying machines and printers using color electrophotographic methods and multifunctional processing machines containing such devices and facsimiles are becoming increasingly popular, but it is generally difficult to use a releasing agent such as wax or the like, for obtaining a suitable level of glossiness of image during reproduction of color image and an excellent transparency when forming an OHP image. For this reason, a great amount of oil is applied onto the fusing roll for facilitating exfoliation, and it becomes difficult to prevent a sticky feeling on the reproduced image, including the images on OHP sheets and to write additional characters or images onto the reproduced image, for example, with a pen or the like. In addition, the use of oil often results in uneven glossiness of the reproduced image. It is more difficult to use commonly-used waxes, such as polyethylene, polypropylene, or paraffin in normal black-and-white copying machines, because the wax impairs the transparency of the resulting OHP images.
As advances in digitalization and high-level image-processing technology proceed, there exists a need for advanced methods for improving image quality further, and in particular, an urgent need for improvement in color image quality, accompanies the rapid popularization of color image-processing machines.
Even if transparency is neglected, it is difficult, for example, to suppress exposure of wax on the toner surface in the method for producing toner in the conventional blending and pulverizing process, and thus such a toner shows marked deterioration in fluidity and causes filming on the developing machine or the photoreceptor when used.
As a basic method of overcoming these problems, a polymerization production process of controlling the exposure of wax on the surface by enclosing it inside toner is proposed, specifically, a method of producing a toner by direct polymerization of a dispersion prepared by dispersing an organic phase containing raw resin monomers and a colorant in an aqueous phase.
Alternatively, methods of producing a toner by an emulsification-polymerization flocculation method are also proposed as means of enabling deliberate control of the shape and surface structure of the toner (Japanese Patent Application Laid-Open (JP-A) Nos. 63-282752 and 6-250439). These are generally processes of producing a toner by mixing a resin particle dispersion prepared, for example, by emulsion polymerization with a colorant particle dispersion containing a colorant dispersed in a solvent, forming aggregates having a diameter similar to that of the toner particle, and fusing the agglomerates by heating.
These production processes allow not only enclosure of wax, but also easier reduction in the size of toner and thus images higher in sharpness and resolution.
As described above, for providing a high-quality image in the electrophotographic process and stabilized performance of toner under various mechanical stresses, it is quite important to optimize the kind and amount of pigment and releasing agent used, inhibit exposure of the releasing agent on the surface, improve the glossiness of printed images by optimizing the resin properties and the releasing property without using fusing oil, and control hot offsetting.
Alternatively, there is also a strong demand for reduction in energy consumption from the environmental point of view, and low-temperature fusing, i.e., image fusing at a lower temperature, is desirable for reduction in the energy consumption of copying machines and printers.
In the methods of performing low-temperature fusing with a toner, resins having a lower glass transition point are generally used as the toner binder resins. However, although a toner from the binder resins having a lower glass transition point is superior in low-temperature fusing efficiency, it is markedly poor in toner stability including toner storage stability, cohesion within the developing machine, and adhesiveness. In addition, the image forming film affixed, for example, on paper is fragile and causes defects easily by abrasion.
Methods of using a crystalline resin are also proposed as the means for achieving the low-temperature fusing (see, for example, Japanese Patent Application Publication (JP-B) No. 4-24702 and Japanese Patent Application Laid-Open (JP-A) No. 9-329917). These methods enable reduction of the fusing temperature, but cause a problem in that it is difficult to obtain a uniform and high-density image because of penetration of the toner fused during fusing into the paper.
Alternatively proposed are numerous methods of using a crystalline resin and an amorphous resin in combination, not of using a crystalline resin alone, as the binder resin (see, for example, JP-A No. 2-79860). Also disclosed are methods of using a polymer prepared by chemically binding a crystalline resin to an amorphous resin (see, for example, JP-A Nos. I-163756, 4-81770, and 4-155351). However, when there is more amorphous resin present than crystalline resin, the amorphous resin represents the continuous phase while the crystalline resin represents the dispersion phase; and in such a case, the melting point of the entire toner is dependent on the softening temperature of the amorphous resin, making low-temperature fusing difficult. Use of a crystalline resin higher in plasticity with an amorphous resin for low-temperature fusing results in deterioration in toner stability similar to when a resin having a lower glass transition point is used as the toner binder resin, decreasing the strength of image film and causing stains and defects by abrasion in the image film.
Conversely, if there is more crystalline resin present than amorphous resin, it is not possible to obtain the advantageous effects of the additional use of an amorphous resin.
Also proposed for low-temperature fusing are methods of using a wax having a low-melting point (see, for example, JP-A Nos. 4-107567 and 8-114942). These methods, which provide a releasing property by using a wax as a releasing agent and allowing release of the wax melted during fusion onto the image surface, often result in offsetting due to a decrease in the melt viscosity of the wax, and are effective only at a temperature sufficiently higher than the melting point of the wax. In addition, the release of wax is dependent on the compatibility between the binder resin and the wax as well as the melt viscosity of the binder resin, and thus, these methods are still unsatisfactory as means for achieving low-temperature fusing.
Especially in recent years, it is desired that power supply to a fusing device be controlled in the standby mode for thorough energy-conservation. Thus, it is necessary to raise the temperature of a fusing device instantaneously from the stand-by mode to a temperature that allows fusing by increasing the power supply to the fusing device before starting image fusing.
It is desirable to reduce the heat capacity of a fusing device to the minimum for that purpose, but in such a case, a fluctuation in the temperature of the fusing device may be expanded to a range larger than before. In other words, the overshoot of the temperature of the fusing device immediately after power is supplied is increased, and the decrease in temperature by the passage of paper is also enhanced. If paper smaller in width than that of the fusing device is fed repeatedly, the difference between the temperatures in the paper-passing and non-passing areas becomes enlarged. In particular, high-speed copying machines and printers, which do not have a sufficiently large electrical capacity have a strong tendency towards the phenomena described above when the heat capacity is reduced. Accordingly, there exists a strong need for a so-called wide fusing-latitude electrophotographic toner that can be fixed at a low temperature and does not cause offsetting even at a temperature in a higher temperature range.
Use of a crystalline poly-condensation resin that exhibits a sharper melting behavior with respect to temperature as the binder resin for toner is known to be effective for lowering the fusing temperature of toner. However, crystalline resins are more resistant to pulverization in the melt-kneading pulverization process and thus often cannot be used.
If a crystalline resin prepared by poly-condensation is used as the binder resin, a reaction demanding stirring at high power at a high temperature of more than 200° C. under an extremely low pressure over a period of 10 hours or more is needed for polymerization, which results in a great amount of energy consumption. In addition, such a resin production facility often demands increased durability and thus a vast amount of facility investment.
As described above, when a toner is produced by the emulsion polymerization aggregation method, the toner may be produced by producing a crystalline poly-condensation resin by polymerization, converting it into a latex by emulsifying the resin or aforementioned compound in an aqueous medium, mixing and coagulating it with a pigment, a wax, and the like, and fusing the resulting aggregates.
However, this method requires an extremely inefficient and energy-consuming step, for example, of emulsifying the poly-condensation resin under high shearing force at a high temperature exceeding 150° C., or dispersing a low-viscosity solution of the resin in an aqueous medium, and then removing the solvent.
Because of the difficulty in avoiding the problem of hydrolysis during emulsification in an aqueous medium, there are always uncertainties in material design being inevitably generated.
Although these problems are more obvious during use of crystalline resins, the same problems occur not only during use of the crystalline resins but also during use of non-crystalline resins.
For example, a method is proposed of producing a toner by forming a fused raw toner by heating and melting the raw toner materials containing at least a polyester resin, forming resin particles by emulsifying the fused toner in an aqueous medium, and aggregating and then fusing the resin particles together with other components (JP-A No. 2002-351140).
When producing a toner according to this method, the binder resin is prepared and emulsified, for example, as follows: First, a polyester having a weight-average molecular weight of approximately 5,000 to 90,000 is prepared by using a conventional poly-condensation catalyst such as tetrabutyl titanate, trimellitic anhydride (TMA) as polyvalent carboxylic acid monomer, terephthalic acid (TPA) and isophlithalic acid (IPA) as bivalent carboxylic acid monomers, polyoxypropylene (2.4)-2,2-bis(4-hydroxyphenyl) propane (BPA-PO) and polyoxyethylene (2.4)-2,2-bis(4-hydroxyphenyl) propane (BPA-EO) as aromatic diol monomers, and ethylene glycol (EG) as an aliphatic diol monomer; allowing the raw materials above to react with each other at 220° C. under high atmospheric pressure and a nitrogen stream for 15 hours; and continuing the reaction at a gradually reduced pressure and finally at a pressure of 10 mm Hg.
Then, this polyester is melted and kneaded with colorant, wax and the like. The resulting mixture is heated to 190° C., and placed in a Cavitron CD1010 dispersion emulsifier (Eurotec, Ltd.). A 0.5 wt % dilute ammonia water is heated to 160° C. with a heat exchanger, and introduced into the Cavitron at a rate of 1 L per minute. After dispersion processing is done, the mixture is cooled to 60° C. to obtain a resin particle dispersion.
For preparation of a toner, the dispersion is further aggregated, fused, washed and dried. Such a method demands a vast amount of energy in the resin production and emulsification processes.
A crystalline resin prepared by poly-condensation exhibits a sharp-melting behavior with respect to temperature and is thus effective for the purpose of low-temperature fusing. On the other hand, amorphous resins are occasionally superior in mechanical strength and consistency of electrostatic properties of the toner when used for an extended period of time.
Accordingly, it is important to improve both the low-temperature fusing efficiency and the reliability of toner when used for an extended period of time, by using not only a crystalline resin but also a non-crystalline resin on the surface of or inside the toner.
In particular, low temperature-fusing toners often cause filming on the photoreceptor and have difficulty in preserving image quality because of a deterioration in charging properties as a developer when used continuously in a summertime environment.
In this case, a commonly used method of preparing toner is mixing, aggregating, and fusing the crystalline resin particle dispersion and non-crystalline resin particle dispersion in water. However, because the thermal fusion characteristics of each resin particle greatly differs in this case, a variation in adhesive strength between particles is generated sometimes resulting in uneven particle distribution during aggregation; and even if toner successfully prepared, the toner's low-temperature fusing properties and charging properties over time are sometimes insufficient because resin particles inside and on the surface of the toner are not distributed as expected.